This invention relates to alternating current (AC) voltage to direct current (DC) voltage conversion circuits and particularly to circuits for regulating input voltages that have peak amplitudes which may vary for providing a constant magnitude, minimal ripple amplitude supply voltage. Due to the difference between power distribution systems employed by different nations, applied line voltages may vary from as low as 90 volts, as used in Japan, to as high as 250 volts, as encountered in Europe. In the United States there is about a 20 percent variation in the nominal voltage on a 120 volt line.
As a result of these voltage variations, a problem may be encountered by the international traveller having a small appliances, such as either an electric shaver or a radio, if the working voltage of the driving or power supply components in such appliances is exceeded. Some electric shavers contain bulky AC wound motors that are used to drive the shaver heads. Presently, it is sometimes necessary to manually switch the shaver to thereby tap a correct proportion of the motor windings in correspondence to the particular line voltage being used. The burden is on the user to see to it that this switching is properly performed in accordance with the different line voltages that may be encountered while travelling worldwide. If the traveller does not correct for a specific line voltage, the shaver motor may be destroyed by excessive currents.
The AC motor referred to in the previous paragraph also presents a production disadvantage in the manufacture of the electric shaver. By nature, AC motors are bulky and expensive. It is, therefore, desired to use small inexpensive direct current motors with permanent magnet fields requiring minimum working voltage. An AC regulator circuit capable of providing a high degree of regulation with varying amplitudes of input line voltage is required to facilitate the use of such DC motors.
Although present AC-DC regulators are adequate for regulating input voltages that have amplitudes that vary from 10 to 20 percent, these regulators are not suitable to be used in small appliances in which the peak amplitude of the input line voltage may vary more than 100 percent. To maintain nominal working voltages of 100 volts in environments having line voltages with amplitudes of 250 volts, present regulators generally have to conduct current of large magnitudes. To withstand the resulting power dissipation, it is necessary to use high power semiconductor devices and heat sinking to provide protection to such devices. This undesirably increases the physical size of such regulators. Moreover, power devices are expensive and provide disadvantages in a marketing environment that demands minimal production costs.
Another problem common to most present regulators is that of transients. Transient voltages can occur during either connection or disconnection of the regulator to or from the line voltage supply. If the transient voltage causes a false triggering signal to occur simultaneously with the peak value of the applied line voltage, the output voltage of the regulator may exceed the rating of the motor and destroy it.
Thus, a need exists for a regulator circuit providing a constant output voltage and which is operable in environments in which extreme amplitude variances in line voltages occur and which does not require manual switching techniques to protect the circuit. A need also exists to develop a regulator circuit which is not damaged by false triggering in response to transients.